Lung volume testing in the elderly

Our population is ageing. As the proportion of elderly increases, it is increasingly important for health- care professionals to be aware of the limitations of some of our most trusted investigations in evaluating the health of our elderly patients.

A paper in the Blue Journal from the start of the month outlines this issue with clarity with regard to lung volume testing. We perform lung volume testing regularly on new patients referred to our service since we obtained equipment 18 months ago to allow us to do this accurately by plethysmography. When a patient performs this maneuver their results are compared against an expected normal range. This ‘normal range’ is calculated using equations which take into account age, height, weight and some derivative measures (such as body surface area and body mass index). These complex equations are pre-loaded in the software we use.

Until now, however, no-one has ever created reference equations for populations of patients aged over sixty-five.

This recent study, from Madrid, took a population of white patients aged between 65 and 85 years who were free of pulmonary or cardiac disease and who had never smoked. Around 320 of these participants peformed lung volume testing, using two different techniques (plethysmography, and helium dilution). Their results were compared against a ‘normal range’ based on equations from a middle aged population. The results from the different techniques were also compared.

Firstly, significant variability between the two modes of testing was found within individuals. Although this was not a surprising finding to anyone who does lung function testing, it is a reminder of the importance of comparing apples with apples. We should not compare lung volume results obtained with dilution techniques to results obtained with plethysmography in any one individual. (As an aside, dilution measures of lung volumes are more difficult to peform in any case and seem to only become more difficult if pathology is present – ie when they are really important. Plethysmography is quicker and more accurate).

The second finding was that the ‘extrapolated’ equations from middle-aged adults overestimated normal and therefore suggested many participants, who had been screened and found to be free of disease, to be below the normal range. For total lung capacity, for example, (TLC is one of the results obtained from lung volume testing) middle-aged reference equations found that between 11.8% and 55.5% of the women and between 14.1% and 25.9% of the men in the group were below the lower limit of normal – when really no more than 2.5% of the healthy population should be in that position.

We need to think about putting these reference equations into our equipment for use in patients in this age range. Meanwhile we need to be cautious about inferring to much from unexpectedly ‘abnormal’ results in older patients.